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MythTV will do it. It's free and runs on linux. You can download just the myth program, or a custom combo of knoppix and myth. There are forums specifically for this, with info on which tuner cards work on linux, etc.
I think there are ones that run on Win, but not sure if any are free.

Your question made me look around there myself and I realised there's no link for KnoppMyth. Here's the link.

When I looked around 6 months or so ago, it seemed like the Hauppage PVR-250 cards were the best bet for this program -- decent quality and linux drivers. You can run more than one and record more than one show at the same time.